Many wireless mesh networks have the following characteristics in common: highly dynamic, autonomous, peer-to-peer, multi-hop, limited bandwidth and computing power, etc. Wireless mesh networks are highly dynamic for two reasons. First, the routers themselves may move (e.g. in mobile or hybrid wireless mesh networks), generating quick response times to topological changes. Second, even if the routers themselves do not move (e.g. in fixed wireless mesh networks), the radio link qualities can change very quickly because of interference, geographical and environmental factors. Traditional routing protocols (e.g. OSPF, RIP), designed for wired infrastructures, cannot handle such quick changes in signal strength. Many of the ad hoc routing protocols (e.g. AODV) lack the ability to flexibly adapt to radio link quality changes.
Wireless networks are becoming increasingly common in peoples' lives. The most popular wireless networks to date are mobile phone networks. Such networks are typically broken down into a number of “cells,” as indicated by the term “cellular phone networks.” Each cell has a set of antennae that send and receive signals to and from cell phone users. Communication lines interconnect the cells to form a cellular network. A commonly-used communication line is a fiber optic line, though alternatives are widely accepted in the field.
In a cellular network, when a user moves from one cell to another, their mobile phone call can continue communication only if a connection to one or more antennae is maintained. The process of transferring a user's connection from one cell to the next is called “roaming.” Roaming is essential to enable a user to stay connected while moving across different cells without dropping calls.
Cellular networks are traditionally not built as “mesh,” but in some form of hierarchical topological. (e.g. a tree topology, or a set of trees) A mesh, on the other hand, is a network where every cell is virtually connected to every other cell in the network. An example of a mesh is a grid map or the Internet. A wireless mesh network consists of cells interconnected by wireless backhaul such that each cell is in virtual communication with every other cell. Wireless mesh networks can also have gateways connected to wired networks. With the popularity of IEEE 802.11 wireless devices, wireless mesh networks have become increasingly popular. An 802.11 mesh network can incorporate inexpensive and relatively low powered 802.11 devices (access points, routers, etc) to form a high throughput and reliable distributed network. The advantages of such a network are analogous to those in distributed PC networks, which generate enormous computing power beyond that obtained with independent PC's.
Roaming is the process by which a user switches between cells in a wireless mesh network. In a large wireless mesh network capable of handling Voice-Over-IP (VoIP) traffic, a roaming solution needs to be scalable, reliable, and provide fast handoff between different cells. Such a roaming solution should scale to at least thousands of network cells, though the limit for future applications is unpredictable. The key to scalability is lower roaming overhead, reducing the burden on the system at each handoff. The wireless mesh network should also be resilient to cell failures in a way similar to the Internet. This resiliency requires the network to take full advantage of the connectivity across the entire mesh. At present, the latency needs to be less than 50 ms to be undetectable to VoIP users.
Existing roaming solutions fall short of these requirements. Roaming solutions in mobile cellular networks cannot be applied because they require hierarchical tree-like topologies. A recent proposal termed “micro-mobility” utilizes solutions such as the Cellular IP and Hawaii protocols derived from cellular phone networks. Micro-mobility solutions are typically designed for a strictly hierarchical network. A hierarchical network introduces bottlenecks due to poor load balancing and is vulnerable to cell failure, among other problems. The main weakness of hierarchical networks, though, is the lack of scalability. Hierarchical networks require significant cache and exchange state information storage among duplicative cells in the system. The amount of information exchanged and maintained grows exponentially with the size of the network and with the amount of roaming activities among users. Thus, such a hierarchical network is not scalable, and becomes impracticable as the number of cells increases.
Another prominent solution (and IEEE standard) on IP networks is called Mobile IP, created by the Internet Engineering Task Force. Mobile IP is scalable, but can suffer from long handoff latency due to the way it forwards data to a mobile user. Optimization techniques to reduce handoff latency require changes on client software, which could be a serious impediment to user adoption.
Prior wireless mesh networks enable a user to switch between mesh routers. When a user device senses that a stronger signal is available than the one currently being used to access the network, a roaming process enables the device to switch to the stronger signal. To roam, the device must contact a cell, sometimes called a home agent, that contains the user's roaming profile. A home agent is typically a server, and resides somewhere within the entire network. To contact the home agent, the user device may have to travel long paths through a multitude of cells. Further, each time the user roams, the home agent must be contacted. This introduces long delays and signal degradation from the users end.